Cancer cells have been shown to express a variety of specific biomolecules such as peptide-receptors, which may serve as recognition sites for a wide range of circulating vectors, as for example peptide-ligands. In case the expression of the target-receptor is higher on malignant cells than in surrounding healthy tissue, the opportunity arises to exploit the interaction between these two molecular entities. For diagnostic imaging or targeted therapy applications, a natural peptide-ligand could be modified to stably bind a diagnostic or a therapeutic radionuclide, e.g. a radiometal or a radiohalogen.
In many cases, a bifunctional chelator is covalently coupled via a carboxyl-functionality to the N-terminal amine of the peptide-ligand to form a peptide bond. In order to increase the biological stability, hydrophilicity, receptor binding affinity and/or internalization efficacy, further modifications of native receptor ligands are attempted, such as strategic amino acid replacements in the peptide chain. Alternatively, introduction of suitable spacers between the chelator and the peptide receptor recognition site or hetero/homo peptide-multimerization may equally lead to advantageous improvements of many biological parameters eventually improving overall pharmacokinetics and target accumulation of the radioactive probe.
The resulting peptide-chelate conjugate after labeling with a diagnostic or a therapeutic radionuclide (radiopeptide) is administered to the patient. The radiopeptide selectively accumulates on cancer-site(s) through specific interaction with the target-molecule, i.e. its cognate peptide-receptor, highly expressed on the tumor. In case of a diagnostic radionuclide, the tumor and metastases are then localized by imaging the site(s) where the radioactive decay occurs using an external imaging device. When the peptide-chelate conjugate is labeled with a therapeutic radionuclide, a radiotoxic load is delivered specifically to the primary tumor and its metastases. The therapeutic radionuclide will then decay on the cancer site(s), releasing corpuscular energy to kill or to reduce (the growth of) the lesions.
This strategy has been elegantly exploited in the area of somatostatin and its receptors. The latter are abundantly expressed in a variety of human tumors, and especially in neuroendocrine tumors (NETs). The advent of OctreoScan® ([111In-DTPA]octreotide) in clinical practice for the successful diagnostic imaging of NETs was soon followed by many new improved somatostatin analogs labeled with a wide range of medically relevant radiometals useful not only for conventional imaging with a gamma-camera, but also for PET and, most importantly, for radionuclide therapy. Ongoing clinical trials have revealed the therapeutic efficacy of these new radiopeptides.
Peptide-receptors and their ligands have emerged as attractive molecular tools in cancer diagnosis and therapy. For example, high density expression of gastrin releasing peptide receptors (GRPRs) has been documented in several frequently occurring human tumors, such as in prostate cancer, mammary carcinoma and lung cancer. As a consequence, GRPRs have lately been gaining momentum as preferred molecular targets for radiolabeled bombesin-like peptides with the aim to upgrade the diagnostic and therapeutic arsenal of nuclear oncology.
Bombesin (BBN) is a tetradecapeptide initially isolated from the skin of the European frog Bombina bombina. Bombesin and its related peptides affect thermoregulation and food-intake after binding to specific receptors in humans. These receptors comprise three subtypes in mammals, the neuromedin B receptor (NMBR or BB1R) with a high affinity for NMB, the GRPR (or BB2R) with a high affinity for GRP and the BB3R, which is an orphan receptor with no-known ligand identified yet. Amphibian BBN binds to NMBR and GRPR subtypes with a high affinity. NMB and GRP are the mammalian counterparts of amphibian BBN and are all related in structure.
Most radiolabeled BBN-like peptides developed for molecular imaging and radionuclide therapy of human tumors have been based on native BBN, or on its C-terminal octapeptide fragment still able to bind the GRPR. These analogs modified as detailed above typically exhibit agonistic properties and internalize in the intracellular region of malignant cells after binding to the GRPR. This property translates into a high accumulation of the radiolabel in the GRPR+ lesions, thereby enhancing either diagnostic sensitivity or therapeutic efficacy.
Unfortunately, BBN-like peptides are potent GRPR-agonists, eliciting adverse effects related to gastrointestinal motility and thermoregulation when intravenously (iv) administered in human even in small amounts. In addition, BBN-like peptides are mitogenic. The above properties have restrained the thorough clinical validation and/or the eventual commercial exploitation of a few promising agonist-based radiolabeled bombesins. This is particularly relevant in the case of targeted radionuclide therapy whereby higher peptide amounts need to be iv administered in patients.
Unlike radiolabeled BBN agonists, radiolabeled somatostatin-agonists, which internalize equally well into somatostatin receptor-expressing malignant cells, do not elicit undesirable physiological effects after iv injection in humans. This fact has fostered the extended and systematic clinical validation of a few promising radiolabeled somatostatins even in the domain of radionuclide tumor therapy.
The radiotracer ([99mTc]Demobesin 1, [99mTc—N4′]DPhe-Gln-Trp-Ala-Val-Gly-His-Leu-NHEt) is known and used in mice bearing human prostate cancer PC-3 xenografts, where [99mTc]Demobesin 1 showed exceptionally superior pharmacokinetic properties as opposed to similarly affine bombesin-based agonists, as for example [99mTc]Demobesin 3-6. Besides its significantly higher tumor accumulation, [99mTc]Demobesin 1 cleared very rapidly from the body of mice and the pancreas, a strongly GRPR-positive organ.
Although first studies in a limited number of prostate cancer patients verified the excellent tolerability of the radiotracer, they revealed a sub-optimal pharmacokinetic profile in humans preventing a further expanded clinical application as a diagnostic imaging tool. More specifically, [99mTc]Demobesin 1 despite its rapid body and pancreas clearance and its rather good in vivo stability, exhibited insufficient retention in malignant lesions in humans as compared to radiolabeled BBN-like agonists. Furthermore, [99mTc]Demobesin 1 was designed for diagnostic imaging using conventional gamma camera or SPECT and is unsuitable for PET or radionuclide therapy applications. Although labeling with the PET radionuclide 94mTc is feasible by means of the acyclic N4-system, the medical use of this radionuclide is restricted both by sub-optimal nuclear characteristics and inconvenient production. On the other hand, therapeutic options are restricted to 186/188Re, as the N4-chelator cannot stably bind most of bi- and trivalent radiometals used in nuclear medicine.
It is therefore the object of the present invention to achieve high uptake and retention of a diagnostic and a therapeutic radiolabel selectively to GRPR+-cancer, both primary and metastatic.